When you consider the x-rays they broadcast, small black holes are like jazz musicians riffing to an erratic bebop beat. The x-rays from their giant cousins, however, play a ponderous tune, more like the slow strains of a Bruckner symphony. Now astronomers have discovered that despite the radical differences in tempo, both kinds of black holes share the same basic rhythm. The team claims its findings, presented 9 April at the United Kingdom's National Astronomy Meeting in Bristol, suggest a new way to weigh black holes.
There's no actual music from black holes, but the x-rays they emit vary in intensity over time. This pattern of variability is best known from relatively small black holes, like the one at the center of the Milky Way, that are partnered with a star. The brightness of their emissions changes every fraction of a second, driven by swirls in the gas sucked from the star and swallowed by the black hole. Giant black holes never seemed to show this kind of intense variability in their emissions, but no one had watched the same hole for very long.
Philip Uttley, an x-ray astronomer at the University of Southampton, U.K., and his colleagues have now spent 6 years measuring the x-ray output of giant black holes at the core of a dozen active galaxies. Using NASA's Rossi X-ray Timing Explorer satellite, they found that the x-ray flicker of these black holes indeed varies over time. In fact, it shows the same characteristic variability as those from the black hole-star systems, only much slower. Significantly, this rhythmic variability seems to depend on the size of the black hole: It's a million times slower for black holes that are a million times more massive.
The findings suggest not only that the x-rays are created in the same way in large and small black holes, but also that looking at the time scale of x-ray emissions could be another way to determine the mass of black holes, says x-ray astronomer Christine Done of the University of Durham, U.K. X-ray variability promises a relatively direct weighing method, adds Uttley, one that will work for distant sources and those shrouded in light-blocking gas. Black hole mass, says Uttley, is a "fundamental parameter" in understanding the birth and evolution of galaxies.